Presently, if one wants to accurately assess the concentrations of chemicals or drugs inside a living animal a sample of the blood or tissue to be studied is removed from the animal and taken to an analytical laboratory to have the chemicals of interest extracted and quantified. Typically a first step is a pre-treatment of the sample to convert it to a form more suitable for chemical extraction. In the case of blood this may be by the removal of blood cells and/or some blood components by the preparation of serum or plasma. In the case of a tissue sample this may be by many processes including freezing, grinding, homogenizing, enzyme treatment (e.g. protease or cellulase) or hydrolysis. Subsequently chemicals of interest are extracted and concentrated from the processed sample. For example serum samples may be subjected to liquid-liquid extraction, solid phase extraction or protein precipitation followed by drying and reconstitution in an injection solvent. A portion of the injection solvent is introduced to an analytical instrument for chromatographic separation and quantification of the components. This method produces accurate results with high specificity for the compound of interest, but is time consuming and labour intensive. Also, because of the large number of steps in the process there is a significant chance of errors in sample preparation impacting the results. This method has good sensitivity and selectivity and accuracy for the target compounds but is limited in that the chemical balance the chemicals exist in inside the animal is disrupted during sampling. In many cases this disruption reduces the value of the results obtained, and in some cases makes this technique inappropriate for the analysis. Where the blood volume removed is a high proportion of the total blood volume of the animal, as is commonly the case when mice are used, the death of the animal results. This means that a different animal must be used for each data point and each repeat. By eliminating the need for a blood draw in this case, fewer animals would be required for testing and a significant improvement in inter-animal variation in the results would be achieved.
Alternatively biosensors have been developed for some applications in analysis of chemical concentrations inside animals. In this case a device consisting of a specific sensing element with associated transducer is implanted and produces a signal collected by an electronic data logger that is proportional to the chemicals to which the sensor responds. The main limitations of this type of device are that they normally respond to a spectrum of chemicals rather than having specificity for only one chemical. Of the spectrum of chemicals to which the sensor responds, some produce a greater and some a lesser response. Sensors are also susceptible to interferences where another chemical present in a system interferes with the response produced by the target chemicals. For these reasons biosensors are normally limited in terms of accuracy and precision. Finally biosensors are typically not as sensitive to low chemical concentrations as state of the art stand alone detectors such as mass spectrometers that are used in the above mentioned conventional analysis techniques and in solid phase microextraction. A strength of this technology is that the chemical balance in the system under study is not disturbed.
The in vivo procedure described here is a significant departure from conventional ‘sampling’ techniques, where a portion of the system under study is removed from its natural environment and the compounds of interest extracted and analyzed in a laboratory environment. There are two main motivations for exploring these types of configurations. The first is the desire to study chemical processes in association with the normal biochemical milieu of a living system, and the second is the lack of availability or impracticality frequently associated with size of removing suitable samples for study from the living system. Newer approaches that extend the applicability of conventional SPME technology, where an externally coated extraction phase on a micro fibre is used, seem to be logical targets for the development of such tools. As with any microextraction, because compounds of interest are not exhaustively removed from the investigated system, conditions can be devised where only a small proportion of the total compounds and none of the matrix are removed, thus avoiding a disturbance of the normal balance of chemical components. This could have a benefit in the non-destructive analysis of very small tissue sites or samples. Finally because extracted chemicals are separated chromatographically and quantified by highly sensitive analytical instruments, high accuracy, sensitivity and selectivity are achieved.
With the current commercially available SPME devices a stationary extraction polymer is coated onto a fused silica fibre. The coated portion of the fibre is typically 1 cm long and coatings have various thicknesses. The fibre is mounted into a stainless steel support tube and housed in a syringe-like device for ease of use. Extractions are performed by exposing the extraction polymer to a sample for a pre-determined time to allow sample components to come into equilibrium with the extraction phase. After extraction the fibre is removed to an analytical instrument (typically a gas or liquid chromatograph) where extracted components are desorbed and analysed. The amount of a component extracted is proportional to its concentration in the sample (J. Pawliszyn “Method and Device for Solid Phase Microextraction and Desorption”, U.S. Pat. No. 5,691,206).
To date commercial SPME devices have been used in some applications of direct analysis of living systems. For example they have been applied for the analysis of airborne pheromones and semiochemicals used in chemical communications by insects (Moneti, G.; Dani, F. R.; Pieraccini, G. T. S. Rapid Commun. Mass Spectrom. 1997, 11, 857-862), (Frerot, B.; Malosse, C.; Cain, A. H. J. High Resolut. Chromatogr. 1997, 20, 340-342) and frogs (Smith, B. P.; Zini, C. A.; Pawliszyn, J.; Tyler, M. J.; Hayasaka, Y.; Williams, B.; Caramao, E. B. Chemistry and Ecology 2000, 17, 215-225) respectively. In these cases the living animals were non-invasively monitored over time by assessing the chemical concentrations in the air around the animal, providing a convenient means to study complicated dynamic processes without interference.
The current commercial devices do, however, have some limitations for in vivo analysis inside a living animal. Firstly, the application to chemical analysis inside animals requires greater robustness in both the extraction phase and the supporting fibre core. In addition, most of the extraction phases currently available are better suited for more volatile and less polar compounds. Only one phase is suitable for liquid chromatography (LC) applications (carbowax/templated resin). Analytes of interest that typically circulate in living systems are less volatile and more polar and require LC analysis, so new or modified extraction phases are indicated. The overall dimension of the current device is typically too large for direct in vivo analysis and for direct interfacing to microanalytical systems, the time required for the LC extraction phase to come into equilibrium with chemicals in a sample is relatively long (typically 1 hr or more in a well-stirred sample) and analysis is sensitive to degree of convection in the sample. Also the present SPME devices cannot be conveniently coupled to positioning devices necessary for in-vivo investigation at a well-defined part of the living system.
It is, therefore, desirable to provide a method and a device that allows minimally invasive sampling, quantification or analysis of a biological system.